A hemoglobin scavenger receptor has recently been identified on monocytes and macrophages (Kristiansen, 2001). This receptor scavenges hemoglobin by mediating endocytosis of haptoglobin-hemoglobin complexes. This receptor has also been identified as M130/CD163, an acute phase-regulated transmembrane protein that has been reported to be expressed exclusively on monocytes and macrophages. CD163 belongs to the group B scavenger receptor cysteine-rich superfamily, a family of receptors that includes CD5, CD6 and WC1 which are present on B, T and CD4−8−γδ T lymphocytes, respectively. Complexes of hemoglobin and multimeric haptoglobin exhibit higher functional affinity for CD163 than do complexes of hemoglobin and dimeric haptoglobin.
Previous studies of antibody-mediated crosslinking of CD163 on cultured monocytes have demonstrated that ligation of surface CD163 induces tyrosine kinase—dependent signals resulting in the mobilization of intracellular calcium, inositol triphosphate production and increased secretion of anti-inflammatory cytokines, including interleukin 6 (IL-6) and granulocyte-macrophage colony stimulating factor (GM-CSF) (van den Heuvel et al, 1999).
Hematopoiesis is defined as the production and development of blood cells, including erythrocytes, granulocytes, monocytes, macrophages, esoinophils, basophils, megakaryocytes, B cells and T cells (Wintrobe, 1999). Hematopoiesis occurs as the result of the proliferation and differentiation of hematopoietic stem cells. Hematopoietic stem cells are pluripotent cells which can give rise to the multiple cell lineages found in the blood. Hematopoietic stem cells reside in the bone marrow and their growth, proliferation and differentiation are influenced by both hematopoietic growth factors and the stromal cells within the bone marrow. Stem cells are believed to normally reside in a quiescent nondividing state until stimulated by specific growth factors whereupon they divide and give rise to highly proliferative progenitor cells committed to the production of blood cells of one or more lineages, such as the erythroid, myeloid or lymphoid lineages.
Certain clinical disorders, termed cytopenias, are characterized by the decreased level of a specific cell type in the circulating blood. For example neutropenia is a disorder whereby there is a diminished level of circulating neutrophils. This disorder can be treated by GM-CSF or G-CSF, two different hematopoietic growth factors. However, administration of these growth factors is often associated with a high incidence of adverse side effects. For example, the administration of G-CSF after allogeneic bone marrow transplantation may result in dyspnea, chest pain, nausea, hypoxemia, diaphoresis, anaphylaxis, syncope and flushing (Khoury et al, 2000).
Neutropenia is also associated with AIDS and is currently treated with growth factors (Dubreuil-Lemaire et al, 2000). There are also forms of severe congenital neutropenia (Dale et al, 2000) in which a small percentage of the patients are refractory to the administration of growth factors.
Anemia is the pathological consequence of insufficient hemoglobin to meet the oxygen transport requirements of the body. Historically, certain anemias have been treated with blood or red blood cell transfusions. A variety of complications associated with transfusions makes this treatment undesirable, including hemolytic, febrile and allergic reactions, along with the potential of the transmission of disease. Stimulating the growth and development of erythroid cells (erythropoiesis) is desirable in the treatment of anemia. There are several causes of anemia, which include excessive blood loss, increased red blood cell destruction, decreased synthesis of red blood cells and abnormal production of hemoglobin. Decreased red blood cell production may result from an iron deficiency (either dietary, maladsorption from the gastrointestinal tract, ineffective iron transport or iron utilization by developing red cells), insufficient erythropoietin (Epo) production (kidney dysfunction) or bone marrow failure. Since the erythropoietic activity of the bone marrow is intact in iron and Epo-dependent anemias, such anemias are amenable to iron or Epo therapy, respectively.
Anemia due to iron-deficiencies is typically treated by the oral or intravenous administration of iron. Patients with chronic renal failure typically suffer from Epo-dependent anemias due to the inability of the kidneys to produce Epo. These patients undergo dialysis and 90% are clinically anemic. The traditional treatment for anemia in dialysis patients consisting of multiple blood transfusions has largely been replaced by the administration of Epo. Indeed, ˜88% of all dialysis patients are treated with Epo. One third of patients on Epo therapy develop hypertension, which can generally be corrected using anti-hypertensive drugs. Erythroid progenitors are stimulated by Epo to differentiate into mature red blood cells and synthesize hemoglobin, the main red blood cell protein.
A major limiting factor of Epo therapy is the cost of long term treatment. Typical Epo doses for patients with chronic renal failure are 225 Units/kg/week administered in three doses. Medicare reimbursement for Epo treatment in the U.S. is $10.00 per 1,000 Units, thus the typical cost for a 70 kg patient would be ˜$8,000 yearly. In 1995, 175,000 US patients were on dialysis resulting in a market in excess of $883 million for this indication alone. Costs for this therapy are estimated to be ˜$1.1 billion for 1996. Novel therapies which would reduce Epo requirements for the treatment of anemia would thus be beneficial to the patient and to the healthcare system. The discovery of other agents capable of reducing Epo requirements for the treatment of Epo-dependent anemias would be advantageous.
Furthermore, there are a variety of anemias which do not respond to Epo therapy. Examples of these types of anemia include chemotherapy-induced anemia and anemia of chronic disease, including malignancies. Patients with acquired immunodeficiency can also suffer from anemia, as do AIDS patients being treated with AZT. These types of anemia may be due to ineffective erythropoiesis as a result of either suppressed Epo production or a decreased response of the bone marrow to Epo. Treatment of these types of anemia involves treatment of the primary disorder; however, if the primary disorder cannot be readily treated, then the therapy for the anemia can include red blood cell transfusions. Adverse side-effects of transfusions include acute and delayed hemolytic reactions and the potential of transfusion of transmittable diseases.
In view of the foregoing, there is a need in the art to develop improved methods for increasing the number of blood cells in a patient through the stimulation of hematopoiesis.